We'd all love a 12% spread or maybe even less. Anyway there's a money factor for most people too. A narrow spread will call for a double number of masts and maybe even more booms too.
I have a wife. She doesn't windsurf (yet) but if she did I wouldn't dare to claim that if I was comfortably powered on a 7.2 she would be too heavy for a 6.0
Regarding cammed sails I would only choose them in +8 m2 sizes unless i were pro slalom sailing...